Representing Sound (AQA GCSE Computer Science)

• Computers represent all data in binary, including sound that we record using a microphone (input) or sound that we playback from a speaker (output)
• For this to happen, sound must be sampled and stored

How is sound sampled & stored?

• Measurements of the original sound wave are captured and stored as binary on secondary storage
• Sound waves begin as analogue and for a computer system to understand them they must be converted into a digital form
• This process is called Analogue to Digital conversion (A2D)
• The process begins by measuring the amplitude of the analogue sound wave at a point in time, called samples
• Each measurement (sample) generates a value which can be represented in binary and stored
• Using the samples, a computer is able to create a digital version of the original analogue wave
• The digital wave is stored on secondary storage and can be played back at any time by reversing the process

• In this example, the grey line represents the digital wave that has been created by taking samples of the original analogue wave
• In order for the digital wave to look more like the analogue wave the sample rate and bit depth can be changed

What is sample rate?

• Sample rate is the amount of samples taken per second of the analogue wave
• Samples are taken each second for the duration of the sound
• The sample rate is measured in Hertz (Hz)
• 1 Hertz is equal to 1 sample of the sound wave

In this example you can see that the higher the sample rate, the closer to the original sound wave the digital version looks

• The sampling rate of a typical audio CD is 44.1kHz (44,100 Hertz or 44,100 samples per second)
• Using the graphic above helps to answer the question, “Why does telephone hold music sound so bad?”

What is sample resolution?

• Sample resolution is the number of bits stored per sample of sound
• Sample resolution is closely related to the colour depth of a bitmap image, they measure the same thing in different contexts

What effect do sample rate and sample resolution have?

	Sample rate		Sample resolution
	High	Low	High	Low
Playback quality	⇑	⇓	⇑	⇓
File size	⇑	⇓	⇑	⇓

How do you calculate the size of a sound file?

• Calculating the size of a sound file is carried out with the following formula:
  • Sample rate x duration x sample resolution

Example

Sound Files

(Sample Rate) x (Duration in seconds) x (Sample Resolution)

Size of sound file =      Sample rate 10 Samples per second Duration 30 Seconds Sample resolution 4 Number of bits stored per sample 10 x 30 x 4 = 1200 bits = 150 bytes